Pneumatic brake booster

ABSTRACT

The present invention discloses a brake force booster for automotive vehicles having a control valve which is operable independently of an actuating rod displacing a valve piston, by way of an electromagnet which actuates a third sealing seat. Displacement of the sealing seat in opposition to the actuating direction of the brake force booster permits ventilating the working chamber. To eliminate all pneumatically induced force components in an independent activation, according to the present invention, a permanent pneumatic connection is provided between a pneumatic chamber delimited in the control housing by the valve member and the vacuum chamber, and in that both a second sealing seat which is provided on the valve piston and used to ventilate the working chamber, the third sealing seat, and the valve member in its sealing lip delimiting the pneumatic chamber in the control housing have almost identical diameters.

TECHNICAL FIELD

This invention generally relates to brake systems and more particularlyto a pneumatic brake force booster for automotive vehicles.

BACKGROUND OF THE INVENTION

International patent application WO 94/11226 discloses a brake forcebooster of this general type. The prior art brake force booster suffersfrom the disadvantage of the effect of annular surfaces on the valvemember of the control valve which are delimited by the second and thirdsealing seats, on the one hand, and the boundary of the mentionedpneumatic chamber, on the other hand, especially in the event ofindependent activation by switching on the electromagnets. Thiscondition affects adversely the use of the independently actuatablebrake force booster especially in control operations. One of the annularsurfaces to which a pneumatic differential pressure is applied duringthe independent activation of the brake force booster produces a forcecomponent which counteracts the independent actuating force generated bythe electromagnet and tends to close the control valve. Thisnecessitates compensating the force component by an increase of theindependent actuating force which must be generated by theelectromagnet.

Therefore, an object of the present invention is to disclose measureswhich permit eliminating the disturbing effects of the mentioned forcecomponent.

According to the present invention, this object is achieved because apermanent pneumatic connection between the pneumatic chamber and thevacuum chamber is provided, and in that both the second and the thirdsealing seat and the valve member in its part delimiting the pneumaticchamber in the control housing have almost identical diameters which,preferably, range in a tolerance width of 1 mm. It is preferred that thepneumatic chamber is delimited by a cylindrical part which cooperateswith a sealing lip designed on the valve member.

In a preferred aspect of the subject matter of the present invention,the valve member has two sealing surfaces which preferably have anannular configuration and are arranged one behind the other in theactuating direction, and the first and the third respectively the secondsealing seat are movable into abutment with the sealing surfaces.

The present invention will be explained in detail in the followingdescription of an embodiment, making reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal cross-sectional view, partly broken off, of adesign of the brake force booster of the present invention in theinactive stand-by position.

FIG. 2 is a view of the control assembly of the brake force booster ofFIG. 1 in the full braking position controlled by the driver.

FIG. 3 is a view of the control assembly of the brake force booster ofFIG. 1 in the release position, with the electromagnet switched.

FIG. 4 is a diagram showing the force-and-travel characteristic curvesof the system illustrated in FIGS. 1 to 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The booster housing 1 (represented only schematically) of the vacuumbrake force booster of the present invention which is shown in thedrawing is subdivided by an axially movable wall 2 into a workingchamber 3 and a vacuum chamber 4. The axially movable wall 2 includes adiaphragm plate 8, deep drawn from sheet metal, and a flexible diaphragm18 (not shown) abutting on the plate. The diaphragm, configured as arolling diaphragm, provides a sealing between the outside periphery ofthe diaphragm plate 8 and the booster housing 1.

A control valve 12 is operable by an actuating rod 7 and accommodated ina control housing 5, which is sealed and guided in the booster housing 1and carries the movable wall 2. The control valve 12 is composed of afirst sealing seat 15 provided on the control housing 5, a secondsealing seat 16 provided on a valve piston 9 connected to the actuatingrod 7, and a valve member 10 which cooperates with the two sealing seats15, 16. Valve member 10 is guided in a guide part 21 sealed in controlhousing 5 and is urged to press against the valve seats 15, 16 by avalve spring 22 that is supported on the guide part 21. The workingchamber 3 is connectable to the vacuum chamber 4 through a channel 28which extends laterally in the control housing 5.

By way of a rubber-elastic reaction disc 6 which abuts on the frontalend 42 of the control housing 5 and a push rod 14 which includes a headflange 23, the brake force is transmitted to an actuating piston of anon-illustrated master cylinder of the brake system. The master brakecylinder is mounted on the vacuum-side booster housing half.

A resetting spring 26 (shown schematically in the drawing) is supportedon the vacuum-side end wall of the booster housing 1 and maintains themovable wall 2 in the initial position shown. In addition, a secondcompression spring or piston-rod return spring 27 is provided which issupported indirectly on the actuating rod 7, on the one hand, and on theguide part 21, on the other hand. The force of the spring 27 ensures abias of the valve piston 9 or its sealing seat 16 in relation to thevalve member 10.

To connect the working chamber 3 to the atmosphere when the controlvalve 12 is actuated, a channel 29 which extends in a generally radialdirection is provided in the control housing 5. The return movement ofthe valve piston 9 at the end of a braking operation is limited by atransverse member 11 which, in the release position of the vacuum brakepower booster shown in the drawing, abuts on a stop 38 provided in thebooster housing 1.

As can further be seen in the drawing, the valve member 10 has acylindrical design and includes an annular first sealing surface 44which cooperates with the first sealing seat 15. Valve member 10 furtherincludes an annular second sealing surface 45 which cooperates with thesecond sealing seat 16. Both sealing surfaces are arranged one behindthe other in the actuating direction of the brake force booster and haveidentical diameters. Further, the valve member 10 has a radiallyinwardly arranged sealing lip 13 on its end remote from the sealingseats 15, 16. In the assembled condition of the valve member 10 in thecontrol housing 5, sealing lip 13 sealingly bears against theabove-mentioned guide part 21, thereby delimiting a pneumatic chamber 17in the control housing 5. The above mentioned channel 28 preferablyconnects the pneumatic chamber 17 to the vacuum which is arranged on theside of the valve member 10 remote from the sealing surfaces 44 and 45,is permanently exposed to the effect of the vacuum that prevails in thevacuum chamber 4.

To initiate independent activation of the brake force booster of thepresent invention irrespective of the actuating rod 7, a third sealingseat 24 is arranged coaxially to the sealing seats 15, 16. The diameterof sealing seat 24 corresponds to that of the second sealing seat 16 andto that of the sealing lip 13 which delimits the pneumatic chamber 17.The third sealing seat 24 is operable by way of an electromagnet 20which is favorably accommodated in a housing 25, rigidly connected tothe valve piston 9, and accordingly is displaceable along with the valvepiston 9 in the control housing 5. The electromagnet 20 includes a coil36 accommodated within the housing 25 and an axially slidablecylindrical armature 31. Armature 31 is partly guided in a closuremember 30 closing the housing 25. A force-transmitting sleeve 19 whichcarries the above-mentioned third sealing seat 24 is supported onarmature 31. Interposed between the valve piston 9 and theforce-transmitting sleeve 19 is a compression spring 32 which maintainsthe armature 31 in its initial position where the third sealing seat 24is axially offset with respect to the second sealing seat 16 provided onthe valve piston 9. The closure member 30 guided in the control housing5, by the intermediary of a transmission disc 33, bears against theabove mentioned reaction disc 6 and permits transmitting the input forceintroduced on the actuating rod 7 to the reaction disc 6.

In the design of the brake force booster of the present invention shownin the drawing, electrical switching means 47, 48 are provided which areimportant especially in braking operations where the electromagnet 20 isactuated in addition to the activation by the driver in order to producefull braking irrespective of the driver's wish (so-called brakeassistant function). It is particularly significant that the switchingmeans 47, 48 are actuated during each braking operation. It must beensured simultaneously that the electromagnet 20 is reliably deactivatedupon completion of the braking operation assisted by independent force.The switching means shown include a microswitch 47 which has two switchpositions and is preferably attached to the valve piston 9 or thehousing 25 of the electromagnet 20. The switching means further includean actuating element 48 which actuates the microswitch 47 by atranslatory movement, is sealed and guided in a bore in the controlhousing 5 and cooperates with a stop on the booster housing. The stop isassigned reference numeral 49 and can be formed by a radial collar ofthe rear booster housing half, for example. A compression spring 50 isinterposed between the actuating element 48 and the control housing 5 sothat the end of the actuating element 48 remote from the microswitch 47abuts with preload on the stop 49.

The operation of the independently operable brake force boosterdescribed and illustrated herein is precisely disclosed in theabove-mentioned international patent application. This obviates the needfor a repetition in the present text.

In a full braking position initiated by a driver's actuation, as shownin the FIG. 2 embodiment, the first sealing surface 44 is in abutment onthe first sealing seat 15, thereby interrupting the connection betweenthe two chambers 3, 4 of the brake force booster. Displacement of thevalve piston 9 in the actuating direction produces a slot between thesecond sealing surface 45 and the second sealing seat 16 provided on thevalve piston 9. The slot permits the atmosphere to enter into theworking chamber 3 and, thus, the build-up of a pneumatic pressuredifferential in the booster housing 1.

FIG. 3 shows the condition which occurs after removal of the actuatingforce by the driver, when the electromagnet 20 is switched on. In thiscondition, the function of the first sealing seat 15 is adopted by thethird sealing seat 24 which has simultaneously displaced the valvemember 10 in opposition to the actuating direction, with the result thatbraking initiated by the driver has been assisted. Removal of theactuating force or the driver's wish for termination of braking isidentified by the above-mentioned electrical switching means 47, 48(so-called release switch) which cause deactivation of the electromagnet20. The sleeve 19 returns into its initial position under the effect ofthe compression spring 32 so that both the second sealing seat 16provided on the valve piston 9 and the first sealing seat 15 provided onthe control housing 5 can be closed.

A configuration of the control valve 12 of the independently actuatablebrake force booster according to the present invention, especially byproviding the two sealing seats 15, 16 and the sealing lip 13 withidentical diameters, prevents pneumatically induced force componentsfrom becoming operative upon independent activation of the control valve12 by the electromagnet 20. This is because the effects of the pneumaticpressure differential on the valve member 10 are balanced. Otherembodiments of the valve member 10 are, of course, also possible.Instead of the sealing lip 13 on the valve member 10, a rollingdiaphragm can be used which serves to connect the valve member 10 to theguide part 21.

The diagram illustrating the force-travel characteristic curves in FIG.4 shows the influence of the rating of spring 32 which biases the sleeve19. As can be taken from the drawing, the variation designated by I₁shows a force-travel characteristic curve of the electromagnet 20 whenactuated by a first current value, and the variations characterized byI₂,3,4 correspond to a second, third and fourth force-travelcharacteristic curve of the electromagnet 20 when said is actuated by asecond, third and fourth current value. The variation characterized byII finally represents the behavior of the consumer system which consistsof the armature 31 of the electromagnet 20, the spring 32, the sleeve19, the valve member 10 and the valve spring 22. The first portion AB ofthe characteristic curve II shows the effect of the compression spring32 between the sleeve 19 and the valve piston 9. The force of spring 32must be overcome before the third sealing seat 24 moves into abutment onthe first sealing surface 44 of the valve member 10. The second portionBC which corresponds to a pneumatic maintaining phase, shows a rise inthe force to be generated by the electromagnet 20, by which the thirdsealing seat 24 is urged into the material of the sealing surface 44until displacement of the valve member 10 in opposition to the force ofthe valve spring 22 and, thus, pressure increase in the system commencesin the point of intersection C of characteristic curve II with thevariation I₃. The section CD corresponds to a zone where the consumersystem is stabely adjustable by changes of the current values between I₃and I₄ being conducted to the electromagnet 20. That means, a definedslot can be adjusted between the second sealing seat 16 and the secondsealing surface 45 of the valve member 10 and, thus, a defined gradientof the pneumatic pressure can be adjusted which prevails in the workingchamber 3. The point of contact D of the force-travel characteristiccurve I₄ with the consumer characteristic curve II simultaneouslyrepresents the transition between the stable pressure increase zone andan unstable zone where the force-travel characteristic curve I₄ of theelectromagnet 20 exhibits a considerably greater gradient than theconsumer characteristic curve II, and where the driver is assisted inpanic stops (so-called brake assistant function). A stable pressureincrease occurs in the zone BA of the consumer characteristic curve IIwhere the electromagnet 20 is furnished with current values between I₁,and I₂.

List of Reference Numerals

1 booster housing

2 movable wall

3 working chamber

4 vacuum chamber

5 control housing

6 reaction disc

7 actuating rod

8 diaphragm plate

9 valve piston

10 valve member

11 transverse member

12 control valve

13 sealing lip

14 push rod

15 sealing seat

16 sealing seat

17 chamber

18 rolling diaphragm

19 sleeve

20 electromagnet

21 guide part

22 valve spring

23 head flange

24 sealing seat

25 housing

26 resetting spring

27 piston-rod return spring

28 channel

29 channel

30 closure member

31 armature

32 spring

33 transmission disc

34 chamber

35 extension

36 coil

37 surface

38 stop

39 channel

40 spring

41 sealing lip

42 front part

43 annular chamber

44 sealing surface

45 sealing surface

46 coil

47 microswitch

48 actuating element

49 stop

50 spring

What is claimed is:
 1. Pneumatic brake force booster for automotivevehicles, comprising:a booster housing having its interior subdivided bya movable wall into a first chamber (vacuum chamber) and a secondchamber (working chamber), a control housing accommodating a controlvalve that controls a pneumatic pressure differential which acts uponthe movable wall, the control valve including a first sealing seat thatis provided on the control housing, a second sealing seat provided on avalve piston, and a third sealing seat provided on a sleeve, the sealingseats interacting with a valve member, wherein the control valve isoperable by both an actuating rod and by an electromagnet independentlyof the actuating rod wherein the armature of the electromagnet is inforce-transmitting interaction with the third sealing seat, wherein thevalve member delimits a pneumatic chamber in the control housing on itsside remote from the sealing seats, wherein a permanent pneumaticconnection exists between the pneumatic chamber and the vacuum chamber,wherein the second and the third sealing seat and the sealing lip of thevalve member have substantially identical diameters.
 2. Brake forcebooster as claimed in claim 1, wherein the diameters of the second andthe third seats and the sealing lip of the valve member all fall withina tolerance width of 1 mm of one another.
 3. Brake force booster asclaimed in claim 1, wherein the pneumatic chamber is delimited by acylindrical guide part which cooperates with said sealing lip of saidvalve member.
 4. Brake force booster as claimed in claim 1, wherein thepneumatic chamber is delimited by a cylindrical part to which the valvemember is connected by way of a rolling diaphragm having a mean diameterwhich corresponds to that of the first and the third sealing seat. 5.Brake force booster as claimed in claim 1, wherein the valve member hastwo sealing surfaces which preferably have an annular configuration andare arranged one behind the other in the actuating direction, and thefirst and the third respectively the second sealing seat are movableinto abutment with the sealing surfaces.